The present invention relates to cryogenic vacuum pumping and, more particularly, to an apparatus and method for increasing the adsorbency of adsorbent pumping surfaces. It specifically relates to two-stage cryogenic pumping and an adsorbent panel for such a pump.
In high and ultra-high vacuum pumping, it is common to provide one or more adsorbent surfaces to "pump" certain gases via adsorption. Adsorption is a process by which a surface of a material retains via physical or chemical action, another material which is brought into contact with it. (Adsorption is to be contrasted with absorption in which one material takes another material into the interior of the same. It should be noted that it is adsorption, rather than absorption, when the surface of pores of a porous material retains another material.)
Two-stage cryogenic pumps are used to pump via condensation those gases which condense at a low temperature, and to pump via both adsorption and condensation those gases which pump at an even lower temperature. These pumps typically include one or more condensing surfaces at an initial or first stage for pumping gases, such as water vapor and carbon dioxide, which will condense at the higher temperature, and panels covered with an adsorbing material, such as activated charcoal or artificial zeolite at a second stage to pump those gases, e.g., oxygen, nitrogen, argon, helium, hydrogen and neon, which will be condensed or adsorbed at a lower temperature. These gases are often referred to in the art as "non-condensible" gases. The first stage temperature typically is in the range of 50.degree.-80.degree. K., whereas the second stage is typically in the temperature range of 10.degree.-22.degree. K.
The adsorbent surfaces generally are a single layer of an adsorbent material adhered with a suitable adhesive, such as epoxy, to a panel or other substrate surface. The panel is maintained at the lower temperature and acts to cool the adsorbent material layer as well as condense those gases which do not condense in the first stage of the pump but will condense at its lower temperature.
The capacity of most adsorbent materials is quite limited. In general, a molecule or atom of a gas to be pumped must contact a surface of the adsorbent material before it will be pumped. Once a particular portion of the surface of an adsorbent material has pumped an adsorbent gas, such surface is no longer available to pump additional atoms or molecules of the gas. For this reason, the adsorbent material typically is provided as a multitude of small pieces having microscopic pores to increase the available surface area. These pieces are adhered as a single layer coating on a substrate with an adhesive. The adhesives used in the past for this purpose, e.g. epoxy, can cover and shield or, in other words, passivate a portion of the available surface area of the adsorbent material. One approach considered in the past to increase the capacity of cryogenic pumps is the concept simply of increasing the area within the pump which is coated with the single layer of adsorbent material. The problem with this is that it increases the refrigeration load on the pump. The surface area made available for adsorbency has been optimized in the past with this in mind.
While providing the adsorbent material over an optimum area as a plurality of pieces having microscopic pores is the standard way of dealing with the capacity problem, the pumps available at the time of this invention often require regeneration. In other words, as the surface of the adsorbent material gets "used", the capacity of the pump decreases to such an extent that the adsorbent process is discontinued by the pump operator and the pump is regenerated. That is, the operator terminates communication between the pump and the volume being pumped, and brings the pump to a higher temperature at which the adsorbent material surfaces will release the pumped gases. Such gases are then removed from the interior of the pump by standard techniques. The capacity of two-stage cryopumps available before the instant invention is in the order of 15-20 liters. That is, after pumping this volume of gas, the pumps must be regenerated.